Coating compositions curable with actinic radiation and based on urethane (meth)acrylates are already known and are described, for example, in the international patent application WO2005/120725. There they are used, in particular, for automotive refinish. In that application, of course, they are subject to exacting requirements in terms of scratch resistance and weathering resistance. The scratch resistance of the coating systems obtained from these coating compositions can be improved, as is known, by an increase in the density of crosslinking. A disadvantage of this, however, is the loss of coating-system elasticity that typically accompanies such an increase. Although the highly crosslinked coating systems often do display a very high scratch resistance, they nevertheless tend to crack, owing to the inadequate elasticity, and consequently do not have optimum weathering stability.
The problem of the loss of elasticity was also recognized in German patent DE 197 09 467. That patent subsequently claims, in general, coating materials which in the cured state are to have a storage modulus E′ in the rubber-elastic range of at least 107.6 Pa and a loss factor tan δ at 20° C. of not more than 0.10. The binder is to be selected in accordance with the stated physical criteria of the fully cured coating system. With regard to the chemical nature of the binder, all that is proposed is a broad range of classes of compound, from which individual binders might possibly meet these criteria for the fully cured coating system. Examples specified include (meth)acryloyl-functional (meth)acrylic copolymers, polyether acrylates, polyester acrylates, unsaturated polyesters, epoxy acrylates, urethane acrylates, amino acrylates, melamine acrylates, silicone acrylates and the corresponding methacrylates (page 4, lines 36-38). Preference is given to using binders which are free from aromatic structural units (page 4, lines 38-39). This, in particular, does not include melamine (meth)acrylates, on account of the aromatic triazine ring that is always present in such compounds. Aliphatic urethane acrylates are particularly preferred. No further specific teaching is given on the chemical nature of the binders that are to be employed.
It was an object of the present invention, therefore, to provide coating compositions which are curable with actinic radiation, or thermally and with actinic radiation, and from which it is possible to obtain coating systems featuring high scratch resistance and at the same time featuring high weathering resistance. The coating obtained is, additionally, to conform to the exacting requirements imposed on an automobile coating system with respect in particular to gloss, gloss stability, chemical resistance, stone chip resistance, and strength of adhesion.